1. Field of the Invention
The present invention relates to methods and apparatus for propagating a desired radiation pattern of microwave energy into biological tissue, such as heart tissue. More specifically, the present invention relates to control of microwave heating in heart tissue to ablate arrhythmogenic cardiac tissues at depths sufficient to effectively treat arrhythmias such as ventricular tachycardia while preventing injury due to excessive heating of surrounding heart tissues, fluids such as blood, and adjacent organs.
2. Description of Prior Art
Each year more than 100,000 people die of ventricular tachycardia. The survival rate is only 5% for persons who develop this condition.
The medical problem relates to scar tissue that forms on the heart because of a heart attack. Normal heart tissue conducts electrical impulses that fan throughout the heart to produce the heart beat. However, scar tissue may develop or contain regions in which the electrical conductivity is changed from that of normal heart tissue. Under conditions that may develop after months or years following a heart attack, the combination of scar tissue and living cells within the scar tissue may begin to produce undesired electrical impulses. The undesired or abnormal impulses can be produced at excessively fast rates. The abnormal impulses may then fan throughout the otherwise healthy heart to produce the dangerous rapid pumping by the heart ventriculars called ventricular tachycardia.
The treatment for this condition is the ablation by some means of the arrhythmogenic cardiac tissues found within the scar tissue. As used herein, ablation refers generally to creating a lesion in the biological tissue that results in a cessation of biological functioning of the remaining living or diseased cells in the scar tissue that disrupt normal cardiac rhythms. For instance, thermal ablation refers to heating cells by about 20.degree. C. to the general range of roughly 57.degree. C. to cause them to cease biological functioning. Once ablated or killed, the cells no longer produce the abnormal impulses that can trigger the rapid increase in heartbeat. However, according to the present invention it is not necessary or desirable to vaporize or char cells for ablation purposes because overheating may cause undesirable side effects.
An additional significant problem of ventricular tachycardia, as compared with other types of cardiac arrhythmias, is that the tissues that produce the abnormal impulses may be found throughout a volume of heart tissue that is relatively deep and wide. The scar tissue may be between 0.5 to 1.0 centimeters in diameter and 1.0 to 2.5 centimeters deep. Effecting the necessary ablation or destruction of the cells throughout the large region is difficult due to the requirement of limiting collateral damage to surface tissues, surrounding tissues, and fluids. In fact, overheating or charring could create new scars that may eventually form new regions of arrhythmogenic cardiac tissues. It is also undesirable to boil or vaporize blood. Yet it is necessary somehow to kill or ablate the cells positioned one to two centimeters below the surface tissue.
Another problem of ablating cells deep below the tissue surface to treat ventricular tachycardia is the difficulty of determining when the cells have been destroyed so that the treatment duration need not be longer than necessary. Stopping treatment when the arrhythmogenic tissue is ablated is desirable to reduce the possibility of complications.
Yet another problem relates to the shape of the scar tissue that may vary significantly from case to case. Besides the above listed problems, providing a means of heating that can efficiently focus on the different shapes of the scar tissue is desirable. Such focussing is desirable to ablate the potentially dangerous cells while avoiding unnecessary damage to healthy heart tissue used to effect normal heart functioning.
Presently, perhaps the most effective long term treatment available for ventricular tachycardia is open heart surgery. During surgery, the diseased portion of the heart is ablated, usually by a cold temperature (liquid nitrogen) probe. However, open heart surgery is so physically stressful that it is not a suitable option for most patients. Furthermore, the cost is high and the recovery period is long and sometimes painful. Therefore, for about 80% of patients, open heart surgery is simply not an option.
Transcatheter ablation effects ablation by means of a catheter that the doctor inserts into the heart through a vein. Due to the potential seriousness of cardiac arrhythmias and the limited number of patients that open heart surgery can help, the development of transcatheter ablation has become an important part of cardiac electrophysiology. Generally, the prior art of transcatheter ablation can be classified as follows:
1) high energy direct current pulses; PA1 2) radio frequency alternating current, typically below about 1 Gigahertz, that is continuous, pulsed, or a combination of pulsed or continuous; PA1 3) laser ablation that is presently limited to intraoperative ablation; PA1 4) cryoablation; and PA1 5) chemical ablation.
Direct current ablation has been used, with a certain amount of success, to treat some types of cardiac tachycardia. However, many problems diminish the utility of direct current techniques. Problems of direct current ablation include limited control of energy delivery and a high rate of serious complications. Also, the lesions so produced tend to be too shallow for treating ventricular tachycardia.
Radio frequency ablation provides much better control of energy delivery and lesion size than direct current ablation. Also, the patient complication rate is lower. However, the lesions are typically shallow and therefore deficient for treatment of ventricular tachycardia. Other problems of radio frequency ablation are discussed in further detail hereinafter.
Laser treatment is presently limited to intraoperative endocardial laser ablation procedures and surgical endocardial resection. The complexity of fiber optic technology and the poor flexibility of the energy delivery systems are the major limiting factors in the use of laser transcatheter ablation.
Cryoablation using catheter delivery is in the experimental stage. Pressurized gas systems have safety and delivery problems. Another disadvantage of cryoablation in the treatment of ventricular tachycardia is that the lesions tend to be small and shallow.
Chemical ablation appears to have many disadvantages for treatment of ventricular tachycardia. The disadvantages include a high complication rate, a high level of potential arrhythmogenesis, a complex delivery system, and significant patient discomfort.
Because of the potential seriousness of the problem of cardiac arrhythmias, numerous inventors have attempted to solve various problems related thereto. For instance, U.S. Pat. No. 4,945,912 to E. Langberg confirms that prior art radio frequency ablation produces lesions that are too shallow for treating some types of cardiac arrhythmias. According to Langberg, previous radio frequency instruments deliver about 10,000 times more energy at the transmitter surface than they deliver in tissues 10 millimeters away, thereby resulting in shallow lesions. A solenoid antenna built according to Langberg to operate at less than I Gigahertz would have a heat dissipation ratio at the catheter wall that is only 100 times greater than the heat dissipation in tissues 10 millimeters away. This ratio is a great improvement over the prior art but still leaves ample room for additional improvement. According to the teachings of Langberg, the depth of heating decreases with increasing frequency. Langberg therefore suggests lowering the frequency to obtain deeper heating depths. Subsequent continuation U.S. Pat. No. 5,246,432 to E. Langberg teaches that one reason for decreased depth of heating at higher frequencies is that the electric field for microwave frequency radiation (f&gt;900 Megahertz) attenuates faster due to "skin depth" attenuation.
U.S. Pat. No. 4,641,649 to Walinsky et al. shows a medical procedure for treatment of cardiac arrhythmias using a catheter that includes a flexible coaxial transmission line terminated by an antenna. When the antenna is at the desired location, Walinsky et al. teach to apply radio frequency or microwave frequency electrical energy to the proximal end of the coax to the antenna. The disclosed system uses a 925 Megahertz supply. Walinsky et al. make no further disclosure regarding other frequencies of operation.
U.S. Pat. No. 5,314, 466 to Stern et al. discloses an assembly for steering and orienting a functional element at the distal end of a catheter tube. The functional element has a major axis aligned with the axis of the catheter tube for steering to a tissue site.
U.S. Pat. No. 5,281,217 to Edwards et al. reveals a self-cooling coaxial antenna assembly for a catheter that conducts a pressurized cooling medium along the coaxial cable for absorbing heat. While this method is effective, it is also more complicated than a catheter without the cooling assembly. Finding a less complicated cooling system is desirable.
U.S. Pat. No. 5,272,162 to Edwards et al. shows a method and apparatus for contacting a heart valve tissue with a catheter tip electrode adapted for atrioventricular node mapping and modification. The tip is conformed to rest stably and comfortably on a cardiac valve such as the mitral or tricuspid valve.
U.S. Pat. Nos. 5,222,501 and 5,323,781 to Ideker et al. disclose a closed heart method for treating ventricular tachycardia in a heart infarct patient. The method comprises defining a thin layer of spared heart tissue positioned between the heart infarct scar tissue and the inner surface of the myocardium of the patient, and then ablating the thin layer of spared heart tissue by a closed-heart procedure with an ablation catheter.
U.S. Pat. No. 5,295,484 to Marcus et al. discloses apparatus that employs ultrasonic energy delivered to heart tissue to destroy the heart tissue implicated in the arrhythmia.
U.S. Pat. No. 5,172,699 to Svenson et al. discloses an electrophysically guided arrhythmia ablation system for ventricular tachycardia or other arrhythmias. The system combines a recorder, for the electrical activation time of various parts of the heart for finding an active site of the arrhythmia, with an energy delivery apparatus for ablation of the arrhythmia.
I.E.E.E. Transactions on Biomedical Engineering, Vol. BME-34 No. 2, February 1987, by D. M. Sullivan, D. T. Borup, and O M. P. Gandhi, entitled "Use of Finite Difference Time-Domain Method in Calculating EM Absorption in Human Tissues" describes the FDTD method as applied to bioelectromagnetic problems and demonstrates a 3-D scan of the human torso.
I.E.E.E. Transactions on Biomedical Engineering, Volume 35, No. 4, April 1988, by D. Andreuccetti, M. Bini, A. Ignesti, R. Olmi, N. Rubino, and R. Vanni, entitled "Vee and Polyacrylamide as a Tissue Equivalent Material in the Microwave Range", discloses the use of polyacrylamide gel to simulate biological tissues at microwave frequencies.
Other related references include Critical Reviews in Biomedical Engineering, by K. R. Foster and H. P. Schwan, Volume 17, Issue 1, 1989, entitled "Dielectric Properties of Tissues and Biological Materials" and the book "Field Computation by Moment Methods", by R. F. Harrington, MacMillan Press, 1968.
Consequently, there remains a need for apparatus and methods to produce lesions within heart tissue of sufficient size to be useful in treating ventricular tachycardia without damaging surrounding tissues. Also there is a need for apparatus and techniques for customized heat profiles for other arrhythmias and other medical problems that respond to thermal cell injury. Those skilled in the art have long sought and will appreciate the present invention that provides solutions to these and other problems.